This invention relates to cutting elements for use in rock bits and more specifically to cutting elements which have a body with a canted cutting face on which is formed an ultra hard material cutting layer.
A cutting element, such as a shear cutter as shown in FIG. 1, typically has a cylindrical cemented tungsten carbide body 10. The cylindrical body has a cutting face forming the interface 12. An ultra hard material layer 14 is formed over the cutting face. The ultra hard material layer is typically polycrystalline diamond or polycrystalline cubic boron nitride. The ultra hard material layer typically has a planar or dome-shaped upper surface 16.
Shear cutters are generally mounted in preformed openings 22 on a bit body 18 at a rake angle 20 typically in the order of 10.degree.-20.degree. (FIGS. 2 and 3). These openings have rear support walls 23. The cutters are brazed to the rear support walls. Typically, a 90.degree.-180.degree. portion 24 of the cylindrical body outer surface is brazed to the rear support wall (FIG. 4). The brazed portions of the cutter body and rear support wall are sometimes referred to as the critical brazing area. During drilling, the portion of the cutting layer opposite the critical brazing area is subjected to high impact loads which often lead to crack formations on the cutting layer as well as to the delamination of the layer from the cutter body. Moreover, these high impact loads tend to speed up the wear of the cutting layer. The component 138 of the impact load which is normal to the earth formations is a severe load because it is reacting the weight of the bit body as well as the drill string. A majority of this load is reacted in shear along the interface between the cutting layer and the cutter body. This shear force promotes the delamination of the cutting layer from the cutter body.
To improve the fatigue, wear and impact lives of the ultra hard material layer as well as to improve the layer's delamination resistance, it is common to increase the thickness of the ultra hard material layer. However, an increase in the volume of ultra hard material results in an increase in the magnitude of the residual stresses formed at the interface between the ultra hard material layer and the cutter body.
Because the overall length of the cutter has to remain constant for mounting in existing bits having the preformed openings 22, the increase in the thickness of the ultra hard material layer results in a decrease in the length of the cutter body. Consequently, the cutter body surface area available for brazing is reduced leading to an increased occurrence of cutter fall out during drilling. Cutter retention, is therefore, reduced when the ultra hard material layer thickness is increased.
Other efforts currently being made to improve the fatigue, wear lives as well as the delamination resistance of the cutting layer, include the optimization of the interface geometry between the cutting layer and the cutter body. By varying the geometry of this interface, as for example by making the interface non-uniform, the magnitude of the residual stresses formed on the interface due to the coefficient of thermal expansion mismatch between the ultra hard material layer and the cutter body is reduced.
Currently, there is a need for cutters having improved ultra hard material layer fatigue, wear and delamination characteristics without a reduction in cutter retention.